VHDL - Writing To Registers - vhdl

Question

I want to use four push buttons as inputs and three seven-segment LED displays as outputs. Two push buttons should step up and down through the sixteen RAM locations; the other two should increment and decrement the contents of the currently-displayed memory location. One seven segment display should show the current address (0–F), and two others should display the contents of that location in hexadecimal (00–FF). This is my code to attempt to do this (I haven't implemented the display yet):
library ieee;
use ieee.std_logic_1164.all;
use ieee.numeric_std.all;
entity raminfr is
port (
clk : in std_logic;
we : in std_logic;
do : out unsigned(7 downto 0)
);
end raminfr;
architecture rtl of raminfr is
type ram_type is array (0 to 15) of unsigned(7 downto 0);
signal RAM : ram_type;
signal read_a : unsigned(3 downto 0);
signal a : unsigned(3 downto 0);
signal di : unsigned(7 downto 0);
signal clock : std_logic;
signal key : std_logic_vector(3 downto 0);
begin
U1: entity work.lab1 port map (
clock =>clock,
key => key,
register_counter => a,
value_counter => di
);
process (clk)
begin
if rising_edge(clk) then
if we = '1' then
RAM(to_integer(a)) <= di;
end if;
read_a <= a;
end if;
end process;
do <= RAM(to_integer(read_a));
end rtl;
library ieee;
use ieee.std_logic_1164.all;
use ieee.numeric_std.all;
entity lab1 is
port(
clock : in std_logic;
key : in std_logic_vector(3 downto 0);
value_counter : buffer unsigned(7 downto 0) ;
register_counter : buffer unsigned(3 downto 0)
);
end lab1;
architecture up_and_down of lab1 is
signal value_in_ram : unsigned(7 downto 0);
signal clk : std_logic;
signal we : std_logic;
begin
U1: entity work.raminfr port map (
do=>value_in_ram,
clk=>clk,
we=>we
);
process(clock, value_counter, register_counter)
begin
if rising_edge(clock) then
if (key(3)='0' and key(2)='0' and key(1)='1' and key(0)='0') then
value_counter <= value_counter + "1";
elsif (key(3)='0' and key(2)='0' and key(1)='0' and key(0)='1') then
value_counter <= value_counter - "1";
elsif (key(3)='1' and key(2)='0' and key(1)='0' and key(0)='0') then
register_counter<= register_counter + "1";
value_counter <= value_in_ram;
elsif (key(3)='0' and key(2)='1' and key(1)='0' and key(0)='0') then
register_counter<= register_counter - "1";
value_counter <= value_in_ram;
end if;
end if;
end process;
end architecture up_and_down;
When I try to compile this, I get the following errors repeating over and over:
Error (12051): Project too complex: hierarchy path is too long
Error (12052): Entity "lab1" is instantiated by entity "raminfr"
Error (12052): Entity "raminfr" is instantiated by entity "lab1"
This is obviously due to the fact that I have port mapped each entity in the other, but I don't know any other way to accomplish what I want to accomplish. Can somebody suggest alternatives?

Answer 1

This is a guess since it's not too clear what you want to do. It seems that the problem is one of design : you have a good idea what the end result should do, but it's not clear how to decompose that into components which interact in the simplest way to accomplish the goal.
I am basing this guess on the fact that the active code in "raminfr" stores and loads data independent of the other stuff that has crept in.
So I am going to suggest that "raminfr" be cleaned up as just a memory component WITHOUT any of the other stuff. It can then be embedded in the "lab1" component which handles keys, and stores and displays values from the correct registers. It can also be reused anywhere else you need a memory.
So let's look at raminfr.
entity raminfr is
port (
clk : in std_logic;
we : in std_logic;
do : out unsigned(7 downto 0)
);
end raminfr;
It has a clock, a write enable input, and a data output. But curiously, no address or data inputs! Now, memory is such a standard "design pattern" that deviation from it is probably ill advised, so let's add them...
entity raminfr is
port (
clk : in std_logic;
we : in std_logic;
addr : in unsigned(3 downto 0);
di : in unsigned(7 downto 0);
do : out unsigned(7 downto 0)
);
end raminfr;
Some variants of the memory pattern have other features; read enables, output enables, separate read and write clocks, etc but this simple one will do here.
You can also use generics to customise its size, modifying its data and address bus widths to match. This makes it much more useful and saves a proliferation of similar but different modules...
Let's clean up the architecture to match.
architecture rtl of raminfr is
type ram_type is array (0 to 15) of unsigned(7 downto 0);
signal RAM : ram_type;
signal read_a : unsigned(3 downto 0);
begin
process (clk)
...
end process;
do <= RAM(to_integer(read_a));
end rtl;
Now we can instantiate it in the "lab1" module, connecting up its new ports
U1: entity work.raminfr port map (
addr => register_counter, -- was "a", typo
di => value_counter,
do => value_in_ram,
clk => clk,
we => we
);
and making any supporting changes to the rest of lab1.
This is not the only reasonable decomposition : you could also make "lab1" a simple component without its own storage and bring out other necessary signals as ports. Then you would need a third "top level" entity whose architecture interconnected lab1 and raminfr.